Location determination (LD) systems, such as the Global Positioning System (GPS), the Global Orbiting Navigational Satellite System (GLONASS) and the Low Earth Orbit (LEO) System, receive and analyze timed signals from a plurality of satellites (usually three or more) in order to estimate the location and time offset of an LD signal receiver. The LD signals received may be used to measure pseudoranges from the receiver to each satellite, carrier phases, signal Doppler shifts and the like, but it is usually necessary to receive and analyze these signals from many satellites in order to estimate the location coordinates and time offset or related quantities.
In some situations, acceptable signals from an adequate number of satellites is not available, because of signal interference from an adjacent urban canyon or other structure(s), because of an unfortunate geometric configuration for the contributing satellites, because of partial or full masking of signals transmitted near the horizon, or because of non-availability of signal processing in one or more receiver channels. What is needed is a signal receiver system that can receive and analyze LD signals from fewer than four satellites to estimate location coordinates and receiver time offset, or other relevant variables. Preferably, the system should work with LD signals received from as few as one or two LD signal sources (usually, but not necessarily, satellites) at substantially the same signal receipt time. Preferably, the system should work with an arbitrary satellite configuration, not merely with satellites in equatorial orbits.